The proposed research is to further study the histological properties of the juxtanodal fibers of the canine hearts, using the Masson's trichrome method. Transmembrane potentials of fibers of this tissue also will be studied. The effects of acetylcholine and/or epinephrine on the automaticity of the juxtanodal fibers shall be determined. Also, the relative sensitivity of the juxtanodal fibers, AV node and His bundle to the effects of acetylcholine and epinephrine shall be determined.